The broad purpose of this work is to further understand how microorganisms and vaccine adjuvants via their interaction with antigen presenting cells, such as macrophages and dendritic cells, affect the generation of T cell mediated immune responses via their ability to regulate the production of critical cytokines, such as IL-12 and IL-10. We initially determined that signaling via complement receptor 3 (CR3, CD11b/CD18), which serves as a receptor for a number of infectious organisms and endogenous molecules, such as the complement fragment opsonin iC3b, suppresses the ability of human monocytes to make IL-12 in response to known IL-12 stimuli. These studies suggest that organisms that bind to CR3 in either a complement-dependent or independent fashion have evolved to take advantage of this pathway to avoid the induction of IL-12 dependent T helper 1 (Th1) immune responses that are important for host defenses. Since these initial findings, we have focused our work on this part of this project on the following areas: (1) Understanding the intracellular mechanism of CR3-mediated suppression of IL-12 production. (2) Determining IL-12 production from cells of patients with a genetic defect in b2-integrin expression (leukocyte adhesion deficiency) (3) Examining the role of CR3 activation on IL-12 production in vitro and in vivo, and (4) Determining the ability of antibodies to CR3, as well as CR4, to affect the outcome of ongoing Th1-mediated autoimmune diseases in a mouse model of inflammatory bowel disease. The second aspect of this project examines the regulation of IL-12 production by G-protein signaling. We showed that a primary mechanism by which the family of vaccine adjuvants known as ADP-rybosylating toxins, which include cholera toxin and pertussis toxin work is by regulating IL-12 production and the subsequent induction of Th1 responses via interaction with G-protein coupled surface receptors. CT acts to potently suppress IL-12 by activating Gs-protein coupled receptors, while PT acts to enhance IL-12 production by inhibiting endogenous suppressive Gi-protein coupled receptor signals. In addition, we have demonstrated that certain chemokines and chemoattractans, which act via Gi-protein pathways, can potently suppress the production of IL-12 from monocytes. We are currently exploring (1) the intracellular mechanisms by which chemokines and ADP-rybosylating adjuvants effect IL-12 production, (2) the significance of these novel mechanisms of immunoregulation in vivo, and (3) possibile ways to exploit these findings for the development of new immunotherapies.